1. Technical Field
This disclosure is directed to air conditioning and refrigeration compressor control and more particularly, to variable speed compressors that incorporate continuous variable transmissions (CVTs).
2. Description of the Related Art
Refrigerant systems are utilized in many air conditioning and heat pump applications for cooling and/or heating the air entering an environment. The cooling or heating load on the environment may vary with ambient conditions, and as the temperature and/or humidity levels demanded by the occupant of the environment change. A compressor is used to compress a working fluid (i.e., the refrigerant) from initial (suction) conditions to compressed (discharge) conditions.
In some refrigerant systems, a single compressor is utilized to compress the refrigerant and move the refrigerant through the cycle connecting indoor and outdoor heat exchangers in a closed loop. However, under many circumstances, it is desirable to have the ability to vary the capacity, or amount of cooling or heating provided by the refrigerant system.
To vary the capacity of a compressor, variable speed drives (VSDs) are known for driving compressors at variable speeds in a refrigerant system. By driving the compressor at a higher or lower speed, the amount of refrigerant that is compressed per unit of time changes, and thus the system capacity can be adjusted. A VSD also allows the removal of all unloading hardware from the compressor system. In typical applications involving more than one compressor, such as multiple circuit chillers, multiplexed compressor chillers, refrigeration and compressor racks, a VSD may be used with each compressor to selectively unload compressors as necessary based on system demand. In general, early VSD designs required a constant voltage:frequency ratio. On the other hand, modern inverters within VSDs provide the ability to adjust both frequency and voltage independently of each other, but the voltage:frequency ratio is preset when the VSD is matched with a motor for a given application.
VSDs are expensive and therefore, multiple compressor systems requiring multiple variable speed drives are also expensive. In addition, the need for multiple VSDs adds to the complexity and size of the air conditioning or refrigeration system.
As illustrated in FIG. 1, an exemplary compressor 10 is powered by a hermetic motor 11 which is, in turn, powered by a variable speed drive VSD 12. The VSD 12 supplies a modulated alternating current output having a characteristic output voltage and output frequency. The VSD 12 receives power from a power supply (e.g., 460 VAC, 60 Hz). A gearbox or variable ratio transmission 13 enables the ratio of motor 11 speed to compressor 10 speed to vary. The hermetically sealed casing is shown schematically at 14.
The torque required by the compressor 10 (and thus supplied by the motor 11) will essentially be a function of the load (e.g., the air conditioning load), the saturated suction temperature (SST) and saturated discharge temperature (SDT). A given motor speed is associated with a proportional frequency position on the fixed voltage/frequency curve of the VSD. At a given point on the voltage/frequency curve, however, the current draw of the drive will accordingly be determined by the SST and SDT values. For example, at a given voltage and frequency, if the SDT were to increase suddenly, the torque would increase at a given speed thus necessitate a power increase from the VSD and, accordingly, a current increase. As a result, the operating efficiency of a variable speed compressor equipped with currently available VSDs remains a concern.
A water-cooled chiller is a machine that removes heat from water via a vapor-compression or absorption refrigerant cycle. A vapor-compression water chiller comprises the four major components of the vapor-compression refrigerant cycle: compressor; evaporator; condenser; and some form of metering device. Water-cooled chillers can employ a variety of refrigerants. Chilled water is often used to cool and dehumidify air in mid- to large-size commercial, industrial, and institutional facilities. Chillers can be water-cooled, air-cooled or evaporatively cooled. Water-cooled chillers may incorporate the use of cooling towers which improve the thermodynamic effectiveness of the chiller as compared to air-cooled chillers.
Large tonnage water-cooled chillers (above 2000 tons) typically use open drive centrifugal compressors powered by electric motors. This is due to unavailability of hermetic motors like the one shown at 11 in FIG. 1 that can deliver power levels above 2000 hp (˜1500 kW). Also, the input power for these applications tends to be between 4.16 kV to 11 kV. VSDs for medium (4.16 kV) and high voltage (6.9 kV) compressors are very expensive and consume a lot of space, thereby presenting installation problems. Still further, in the Middle East, the input power for large tonnage water-cooled chillers is 11 kV. VSDs for this voltage are not commercially available and must be custom built.
Accordingly, there is a need for an improved variable speed compressor design that does not rely upon a variable speed drive or a VSD.